The invention relates to a method and apparatus for forming fold lines in substantially hard plastic material.
The use of cooperating embossing and flattening rollers to produce fold lines in substantially flat material is known from U.S. Pat. No. 3,292,513. However, there is no mention therein of heating the sheet blank or the embossing tool. Further, if transparent PVC is embossed using this apparatus, the plastic material in the region of the fold line tends to turn white upon bending of the fold line. In U.S. Pat. No. 4,664,648, the embossing of the grooves that produce the fold lines is done intermittently, the result being a limitation on working speed. Heating can be done to assist in the forming of the grooves, but it is not stated how this is to be done.
German Pat. No. 3,210,967 relates to the manufacture of folding-box blanks from hard PVC sheeting. In this case, the entire folding-box blank consists of transparent PVC sheeting. In the area of the fold lines, the hard PVC sheeting is embossed linearly. To create this embossing, the PVC sheet is first warmed by means of radiant heat directed at the top surface of the sheet; then, just before embossing the grooves that create the fold lines, the sheet is moved along a flow of heating gas aimed at the top surface of the sheet, and its entire surface is heated to a temperature below the melting temperature of the PVC material. The primary goal is to prevent spring-back of the hard PVC when the box is set up. A great deal of heating is required, which always carries with it the risk that the hard PVC sheeting will become rippled and will no longer provide a clean window.
The heating of the entire area of the PVC sheet is avoided in the process according to U.S. Pat. No. 4,179,252, in which the sheet is heated in the area of the fold line or bend edge by means of high frequency. In practice, however, high-frequency heating is inconvenient, because of the necessary expenditure for insulation and protection against stray fields.